It is known to provide electrostatic actuation in micro-electromechanical systems (MEMS) devices that may include an actuator (e.g., a cantilever beam) responsive to such electrostatic actuation. For example, in MEMS switches the electrostatic actuation generally occurs by applying a voltage from a voltage source between a gate terminal and a source terminal in a three terminal device; or between the gate terminal and gate ground for four terminal devices. The actuation voltage can range from approximately 3V to approximately >100V and may be typically applied as a step function, or a realizable approximation of a step function.
For example, when the step function voltage is low (e.g., 0V), a normally open switch would remain open. When the step function voltage goes high (e.g., 100V), the switch would be closed to a conductive switching condition. The implementation of the control for the voltage source tends to be uncomplicated for this type of electrostatic actuation. Metaphorically speaking this would be analogous to accelerating a vehicle (e.g., cantilever beam) as fast as possible (no brakes applied) to reach a post (e.g., a switch contact).
It is also known that this form of electrostatic actuation (e.g., step function) may introduce undesirable effects either during a switch closing event or a switch opening event. For example, in a switch closing event, as the cantilever beam approaches the switch contact, the diminishing gap between the gate and cantilever decreases and causes an increase in the electrostatic force (∝1/gap2) acting on the cantilever. As a result, the cantilever beam greatly accelerates as it approaches the contact and may impact the contact with a substantial force (e.g., high speed impact).
This high speed impact may have several consequences. First, after the initial high speed impact, the beam and/or contact may rebound (e.g., mechanical oscillation or bounce) before being driven by the actuation voltage to establish a continuous contact. This bouncing can occur one or more times before the beam finally settles. Some approaches to solve the high speed impact (and concomitant) bouncing have generally involved cumbersome and costly approaches that can affect the structural design of the MEMS device, e.g., changing the physical dimensions and/or material of the beam to make it stiffer, changing the atmosphere where the switch operates, using a dampening structure, etc. Other approaches have involved lowering the intensity of the actuation voltage to decrease the electrostatic force applied, (metaphorically speaking this may be conceptualized as not accelerating the vehicle as fast as feasible to the post). However, this tends to increase the switch actuation time to an unacceptable level. Another consequence of a high speed impact is a tendency to rapidly degrade the switch contacts over time. The number of operational cycles that a switch is rated to perform over its lifetime is often limited by the wearing of the contacts. For example, if the amount of physical impact on the colliding switch contacts could be reduced, then the amount of bounce would be reduced or eliminated and a substantial number of operational cycles could be added to the ratings of the switch.
Similarly, during a switch opening event, the cantilever beam tends to overshoot its neutral (e.g., normal) open position and may oscillate till it eventually reaches such neutral position. This oscillatory motion may create a varying standoff voltage during the opening event. An oscillatory movement means that even after the MEMS switch has opened and a nominal rated voltage standoff has been reached, it is possible for the switch (e.g., cantilever position) to momentarily fall below its rated standoff voltage one or more times before finally settling at the neutral position and permanently meeting the nominal value for voltage standoff. During a moment when the switch falls below its rated standoff voltage, this may cause the voltage standoff to be less than the required dielectric isolation with respect to the source (load) voltage and may lead to an undesirable arcing (voltage breakdown) condition, or to a momentary re-closure due to electrostatic attraction.
In view of the foregoing considerations, there is a need for an improved electrostatic control. For example, it would be desirable to provide a system and/or techniques for appropriately adjusting (shaping) the gate actuation voltage to reduce the impact of the collision of the cantilever beam in a MEMS device (e.g., a switch) (or reduce oscillatory movement (e.g., overshoot) of the cantilever beam during a switch opening event) without substantially reducing the actuation time of the switch.